The moulds made of cast iron, bronze (and other alloys comprising copper and tin), or steel (notably steels of iron-carbon, stainless steel, refractory steel type) which are generally used in the making of glass objects, have a tendency to wear rapidly, in particular in zones such as the joint plane J (or seam), the bottom F, the ring or instead the neck C of the mould. In fact, during the step of moulding the parison, in this particular case glass, the moulds undergo a phenomenon of abrasion and/or corrosion, on account of the presence of silica in the glass. Furthermore, during a moulding cycle, a mould can undergo strong thermal stresses, passing in a very short time from 400° C. to 800° C.
In order not to have to change the entire mould, hardfacing techniques have thus been proposed, during which a layer of metal alloy is made to melt on the surface of the mould, notably at the level of the edges. The mould thereby obtained is then machined after cooling in order to obtain the desired geometry.
The hardfacing of the moulds is carried out with a blowtorch or by plasma transferred arc (PTA).
Blowtorch hardfacing consists in brazing a powder or a metal wire on the moulding surfaces of the mould. To do this, the mould is preheated to a temperature of the order of 350° C. to 650° C., and one melts with the blowtorch the hardfacing metal as well as a superficial layer of the mould with the blowtorch in order to thereby ensure the bond between the two materials.
However, this hardfacing technique is carried out manually. It thus has several drawbacks, among which the drudgery of the work for the operator (who has to work for twenty or so minutes in an environment close to a temperature near to 650° C. in order to guarantee the brazing of the metals), the necessity of know-how of the operator and a wide variability of results. It may in fact be noted that the success rate does not exceed 90% of the filled parts, and can go down to 25%, on account of disbondments of hardfacing metals.
Furthermore, in order to obtain a geometry similar to the initial geometry of the mould, generally a very large quantity of hardfacing metal is deposited (of the order of double the necessary quantity, or more), then the resulting surface is machined. This method is thus costly in raw materials, and the machining operation is long and fastidious.
Finally, the zone of the mould which has been overheated during the hardfacing process has thermal and mechanical properties different to the rest of the cast iron constituting the mould. In fact, for a mould made of cast iron, in said zone, known as “Heat Affected Zone” (HAZ), the cast iron becomes “white”, in other words very hard and brittle, fragile and difficult to machine. The mould thus becomes more fragile in this zone and is thus likely to break more easily in the case of mechanical or thermal shocks. Yet, in the case of hardfacing by blowtorch, the HAZ is relatively large and has as a general rule dimensions (surface and depth) of the order of 3 to 4 mm.
In order to limit the extent of the HAZ on the finished mould, it has been proposed to carry out a re-crystallisation annealing of the mould for several hours. This step is however rarely implemented because too long and too restrictive.
Plasma transferred arc (PTA) hardfacing for its part follows a similar principle to hardfacing by blowtorch, the source of heat being replaced by a plasma column.
The advantage of this method is that the plasma has a much higher temperature than the flame of a blowtorch (from 15000 to 20000° C.), which makes it possible to heat more rapidly the hardfacing material to be melted and the surface of the part on which it is deposited, thereby restricting the dimensions of the HAZ to around 2 mm.
Furthermore, the success rate of this method is much better than in the case of a hardfacing by blowtorch (of the order of 95%), since it may be automated.
It has nevertheless certain drawbacks. In particular, it is very difficult to apply it to lamellar cast irons (cast irons in which the carbon is in lamellar graphite form) because the application of the method leads to a risk of degassing of the graphite, which can cause defects (for example holes) in the deposit. It will be recalled in fact that the proportion of carbon in a given cast iron is not fixed, but rather an order of magnitude that can vary within a given bracket. Yet, this proportion may have important consequences on the result obtained in the HAZ. Moreover, it remains of important dimensions within the scope of the PTA method.
Furthermore, the quantity of hardfacing metal welded onto the mould remains important compared to the quantity necessary to obtain the desired final geometry, thereby increasing the overall cost of the hardfacing.
Finally, this method necessitates preheating the mould to a temperature of the order of 350° C., which imposes cooling the part by stages after the hardfacing if it is wished to preserve the mechanical and thermal properties of the cast iron, which can last several hours.
The document WO 2009/090622 describes a method of repairing a damaged glass mould, in which a crack has formed for example, during which the whole of the cavity of the mould is machined in order to remove a metal layer from the mould. The mould is then heated to a temperature of the order of 450° C. with the aim of opening the crack and removing a surface layer of metal around the crack, before filling the crack with molten metal and leaving the mould to cool to ambient temperature. The mould is then annealed at a temperature of the order of 600° C., then a hardfacing layer of melted metal is deposited on the whole of the cavity thereof over a thickness greater than that of the metal layer that has been removed during the machining step, for example by depositing a metal powder on the surface of the mould which is remelted and consolidated by means of a laser beam. Finally, the cavity of the mould is machined in order to remove the excess of material.
This method thus makes it possible to repair glass moulds and to extend the lifetime thereof. Nevertheless, it necessitates numerous steps, of which in particular steps of machining the whole of the cavity thereof (twice), preheating, annealing, etc. which are long, fastidious and have a high cost in raw materials and in energy. In addition, a large quantity of hardfacing material is necessary in order to be able to machine the surface of the mould after depositing the hardfacing material, and it is difficult to envisage from the industrial point of view hardfacing the whole of the surface of a glass mould by means of a laser.
The document WO 01/28942 for its part proposes hardfacing the surface of glass moulds during which the cavity of the mould is covered with a protective layer so as to make the internal surface of the mould more resistant to the different moulding cycles. To do this, this document suggests the deposition of a powder of metal alloy having an appropriate composition in the cavity, for example by thermal spraying, then the melting of the powder by heating the mould under vacuum at high temperature. This method has the advantage of extending the lifetime of the mould. Nevertheless, it has a high cost, notably in terms of filler material, and may be difficult to implement according to the powder melting technique used.
In the case where the mould would be all the same damaged despite its protective layer, this same document proposes locally repairing the mould by melting a patch of metal alloy (of the same appropriate composition) on the damaged zone. Nevertheless, according to the means used for melting the patch, it risks disbonding, or at least locally weakening the mould. Furthermore, its metallurgical quality is lower in comparison with that of the rest of the mould.
Thus, current techniques are always very imperfect and do not give entire satisfaction either with regard to their efficiency or their rapidity.